Representative microorganisms used for ethanol production are yeast belonging to genus Saccharomyces or bacteria belonging to genus Zymomonas or Zymobacter. These microorganisms normally produce ethanol efficiently from monosaccharide such as glucose, but are incapable of producing ethanol from oligosaccharide or polysaccharide. In carrying out ethanol production from cellulosic biomass as the feedstock, therefore, it is necessary to first degrade cellulose to monosaccharide which can be fermented by microorganisms. Degradation and saccharification of cellulosic biomass are normally carried out by enzyme process using cellulase or acid saccharification process using sulfuric acid or the like. Whereas, problems are present with these methods such that complete degradation of cellulose to monosaccharide is occasionally found difficult, or excessive reaction to raise the degradation ratio may reduce the sugar recovery and in consequence aggravate ethanol production efficiency.
Accordingly, therefore, for improving yield in ethanol production from biomass feedstocks, it is necessary to introduce β-glucosidase gene into the microorganisms used for ethanol production to construct transformed microorganisms which are capable of producing ethanol on substrate of cellooligosaccharide, a partial decomposition product of cellulose.
Genus Zymomonas and genus Zymobacter are known to show higher fermentation speed than yeast of genus Saccharomyces, and various attempts were made relating to construction of transformed microorganisms using Zymomonas bacteria as host cells. For example, U.S. Pat. No. 5,712,133 disclosed transformation of Zymomonas bacteria to impart thereto pentose fermenting ability. However, when β-glucosidase gene is introduced into Zymomonas bacteria by the method described in said U.S. patent, β-glucosidase is not secreted exocellularly, and furthermore because cellooligosaccharide cannot permeate through cell walls of Zymomonas, fermentation of cellooligosaccharide to ethanol is impossible. WO98/45451 disclosed transformation of cellobiose-incorporating gene of bacteria belonging to genus Klebsiella into Zymomonas to enable intracellular ethanol production from cellobiose, but its ethanol production efficiency is low.